Digestion and Nutrition

Chapter 40

Impacts, Issues

Hormones and Hunger

Fat cells secrete leptin, which reduces appetite; an empty stomach secretes ghrelin, which makes you hungry – the goal is healthy nutrition

40.1 The Nature of Digestive Systems

Digestive system

• A body cavity or tube that mechanically and

chemically breaks food down to small particles, then to molecules that can be absorbed into the

internal environment

• Interacts with other organ systems to maintain

homeostasis

Digestive System Interactions

Incomplete and Complete Digestive Systems

Incomplete digestive system

• A saclike gut with one opening in the body surface for food to enter and waste to leave

Complete digestive system

• A tubular gut with an opening at both ends

• Includes mouth, pharynx, esophagus, stomach, small and large intestines, and anus

Incomplete and Complete Digestive Systems

Five Functions of a Complete Digestive System

1. Mechanical processing and motility

2. Secretion of digestive enzymes into the lumen

3. Digestion of food into absorbable molecules

4. Absorption of nutrients into extracellular fluid

5. Elimination of solid residues

Dietary Adaptations

Bird adaptations

• Size and shape of bills adapted to different diets

• Crops and gizzards

Mammal adaptations

• Teeth adapted to different diets

• Multiple stomach chambers in ruminants

Some Adaptations of Mammalian Digestive Systems

40.1 Key Concepts

Overview of Digestive Systems

Some animal digestive systems are saclike, but

most are a tube with two openings

In complex animals, a digestive system interacts

with other organ systems in the distribution of

nutrients and water, disposal of residues and

wastes, and homeostasis

40.2 Overview of the Human Digestive System

Humans have a complete digestive system lined with mucus-covered epithelium

If the tubular gut of an adult human were fully stretched out, it would extend up to 9 meters (30 feet)

Accessory Organs

Accessory organs along the length of the gut secrete enzymes and other substances that break down food into its component molecules

• Salivary glands

• Pancreas

• Liver

• Gallbladder

From Mouth to Stomach

Food is partially digested in the mouth and forced into the pharynx by swallowing

Food is moved through the esophagus by peristalsis through a sphincter to the stomach, which adds acids and enzymes to food and mixes them together to form chyme

Gastrointestinal Tract

In the small intestine, carbohydrates, lipids and proteins are digested by secretions from liver and pancreas; nutrients and water are absorbed

The large intestine absorbs water and ions, and compacts wastes, which collect in the rectum, and are expelled from the anus

The Human Digestive System

40.3 Food in the Mouth

Digestion begins when teeth mechanically break

down food into smaller bits

Teeth consist mostly of bonelike dentin; the

crown is covered by a hard layer of enamel

Salivary amylase secreted by salivary glands

hydrolyses starch into disaccharides

Four Types of Teeth in Humans

Four Types of Teeth in Humans

40.4 Food Breakdown

in the Stomach and Small Intestine

Carbohydrate breakdown begins in the mouth

and is completed in the small intestine

Protein breakdown begins in the stomach and is

completed in the small intestine

Lipids are digested in the small intestine

Stomach Structure

Digestion in the Stomach

The stomach has three digestive functions

• Stores food and controls the rate of passage to the

small intestine

• Mechanically mixes and breaks down food

• Secretes substances used in chemical digestion

Digestion in the Stomach

Stomach mucosa secretes gastric fluid

containing hydrochloric acid and enzymes that

begin protein digestion

• Gastrin signals secretion of acid and pepsinogens

• Acid unfolds proteins

• Pepsin breaks proteins into peptides

Chyme passes into the small intestine

Digestion in the Small Intestine

In the small intestine, chyme mixes with secretions from the pancreas and liver

Pancreatic enzymes break down larger molecules into units that can be absorbed

• Monosaccharides, monoglycerides, fatty acids, amino acids, nucleotides, nucleotide bases

• Bicarbonate from the pancreas buffers acids so enzymes can work

Digestion in the Small Intestine

Lipid (fat) digestion in the small intestine requires enzymes and bile, which is produced by the liver and stored in the gallbladder

Bile

• A mixture of salts, pigments, cholesterol and lipids that emulsifies fats into small drops that enzymes can break down into fatty acids and monoglycerides

Structure of the Small Intestine

Summary: Chemical Digestion

Controls Over Digestion

The nervous system, endocrine system, and nerves of the gut wall control digestion

Arrival of food in the stomach sends signals to gut muscles, glands, and brain

Sympathetic neurons slow digestion during stress or exercise

Hormonal Controls of Digestion

40.5 Absorption From

the Small Intestine

The small intestine is the main site of absorption

for the products of digestion

Brush border cells that project into the lumen

function in both digestion and absorption

Cells in the intestinal lining secrete digestive

enzymes, hormones, mucus, and lysozyme

Surface Area of Intestinal Mucosa Enhances Absorption

Three features increase surface area:

• The lining is folded

• Multicelled, fingerlike absorptive structures (villi)

with lymph and blood vessels extend from folds

• Brush border cells on the villus surface have

membrane extensions (microvilli) that project

into the lumen

The Lining of the Small Intestine

Water and Solute Absorption

Transport proteins move salts, sugars, and amino acids from the intestinal lumen, into brush border cells, then into interstitial fluid in a villus

Water follows the solutes by osmotic gradient

Capillaries in the villus distribute water and solutes through the body

Fat Absorption

Fatty acids and monoglycerides combine with bile salts to form micelles, which aid diffusion into brush border cells (bile salts stay in lumen)

In brush border cells, fatty acids and monoglycerides combine with proteins to form lipoproteins, which enter the villus by exocytosis

From interstitial fluid, triglycerides enter lymph vessels, which empty into the bloodstream

Digestion and Absorption in the Small Intestine

40.6 The Large Intestine

The large intestine is wider than the small

intestine, but also much shorter—only about 1.5

meters (5 feet) long

The ascending colon begins at the cecum, where

the appendix is attached

The descending colon attaches to the rectum

Structure of the Large Intestine

Function of the Large Intestine

The large intestine completes the process of absorption, then concentrates, stores, and eliminates wastes

Bacteria in the colon make vitamins K and B12, which are absorbed through the colon lining

Stretch receptors in the rectum trigger the defecation reflex

Disorders of the Large Intestine

Diarrhea may result from a bacterial infection, and cause dehydration

Appendicitis must be treated to prevent rupture and infection of the abdominal cavity

Colon polyps leading to cancer can be detected and removed by colonoscopy

40.2-40.6 Key Concepts

Human Digestive System

Human digestion starts in the mouth, continues in

the stomach, and is completed in the small

intestine

Secretions of the salivary glands, liver, and pancreas aid digestion

Most nutrients are absorbed in the small intestine

The large intestine concentrates wastes

40.7 Metabolism of

Absorbed Organic Compounds

Absorbed compounds are carried by the blood to

the liver, which plays a central role in metabolism

Most absorbed compounds are broken down for

energy, stored, or used to build larger compounds

Excess carbohydrates and proteins are converted

to fat and stored in adipose tissue

Liver Function

The liver detoxifies dangerous substances (alcohol, NH3), and stores fat-soluble vitamins (A, D) and glucose (as glycogen)

Between meals, the liver provides the brain with glucose by breaking down stored glycogen

Liver Function

40.8 Human Nutritional Requirements

Eating provides your cells with a source of energy

and a supply of essential building materials

Nutritional guidelines based on age, sex, height,

weight, and activity level can be generated online

at mypyramid.gov

Some USDA Nutritional Guidelines

Energy-Rich Carbohydrates

Good (complex ) carbohydrates provide energy, vitamins, and fiber (soluble and insoluble)

• Fresh fruits, whole grains, and vegetables

Not so good (processed) carbohydrates have “empty calories”

• White flour, refined sugar, corn syrup

Lipids

Lipids are used in cell membranes (phospholipids and cholesterol), as energy reserves, insulation and cushioning, and to store fat-soluble vitamins

Essential fatty acids (linoleic and alpha-linoleic acids) must be obtained from the diet

Good Fat, Bad Fat

Unsaturated fats are liquid at room temperature

• Polyunsaturated fats (such as omega-3 fatty acids) and monounsaturated fats (such as oleic acid) have specific health benefits

Saturated fats (in meat and dairy products) can increase risk of heart disease, stroke, or cancer

Trans fats are worse than saturated fats

Main Types of Dietary Lipids

Body-Building Proteins

Proteins are the source of amino acids used to build all body proteins

Meat provides all eight essential amino acids

Most plant foods lack one or more amino acids, but can meet all human amino-acid needs when combined correctly

40.9 Vitamins, Minerals,

and Phytochemicals

Vitamins

• Organic substances that are essential in very small

amounts in the diet (coenzymes)

Minerals

• Inorganic substances with essential metabolic functions (such as iron in hemoglobin)

Phytochemicals

• Beneficial organic molecules found in plant foods

Major Vitamins

Major Minerals

40.7-40.9 Key Concepts

Organic Metabolism and Nutrition

Nutrients absorbed from the gut are raw materials used in synthesis of the body’s complex carbohydrates, lipids, proteins, and nucleic acids

A healthy diet normally provides all nutrients, vitamins, and minerals necessary to support metabolism

40.10 Weighty Questions,

Tantalizing Answers

Being overweight increases health risks

• Type 2 diabetes, high blood pressure, heart disease, breast and colon cancer, arthritis, gallstones

An unhealthy overabundance of fat (obesity) stresses fat cells, triggers inflammatory response

• Fat cells do not increase in number after birth

• Excess weight overfills existing fat cells

The “Right” Body Weight

Body mass index (BMI) estimates health risks

• Overweight: 25 to 29.9

• Obese: 30 or more

BMI = [weight (lbs) x 703] height (in)2

Weight Guidelines

Genes, Hormones, and Obesity

To maintain body weight, energy (caloric) intake must balance with energy output

Genetic factors influence how difficult it is for a person to reach and maintain a healthy weight

Hormones such as leptin can influence both appetite and metabolic rate

Genes, Hormones, and Obesity

40.10 Key Concepts

Balancing Caloric Inputs and Outputs

Maintaining body weight requires balancing calories taken in with calories burned in metabolism and physical activity